An iron-containing protein orients itself within a magnetic field.

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What do monarch butterflies, salmon, lobsters, bats, mole rats, and marine nudibranch mollusks have in common? As I'm sure you already knew, all of these species (among others) can sense and use magnetic fields.

Although the ability of biological entities to register magnetic fields is fairly well accepted, the means by which they do so hasn't been definitively identified. A lot of attention has focused on cases where small clusters of iron are formed within cells. But researchers in China figured that a protein might exist that could act as a magnetic sensor. So they screened the Drosophila genome for one that fit the bill—and found it.

A couple of different models have been postulated to explain the biological basis of magnetosensing. Cryptochromes (Cry) are light-sensing proteins used by birds to orient and navigate using the Earth's magnetic field. Although these can sense the inclination of the geomagnetic field, they cannot detect polarity and thus cannot function as a compass.

Scientists thus assumed that another protein must be involved, likely one that binds to iron (which can detect polarity and thus can function as a compass). So they scanned a database of the Drosophila genome for genes encoding proteins that (a) bind to iron; (b) are expressed in the head (where Cry is); and (c) are found within the cell, rather than on the cell membrane (again, that's where Cry is). They identified nine candidates in this in silico search, but only one bound to the Drosophila cryptochrome. Bingo: the Drosophila magnetoreceptor protein, dMagR.

Like cryptochromes, genes for MagR are found in all animal species. In addition to Drosophila, Cry/MagR complexes were found in butterflies, pigeons, mole rats, minke whales, and humans. Further examination of the complex purified from pigeon retina revealed that it consists of a linear core of iron-containing MagR proteins surrounded by a sheath of Cry proteins. As a unit, the complex acts as a light-dependent biocompass, capable of detecting the polarity, intensity, and inclination of the Earth's geomagnetic field. It has an intrinsic magnetic moment, as verified by the fact that it orients parallel to an enhanced external magnetic field.

The authors suggest that the coupling of light sensitivity (from Cry) and magnetic detection (from MagR) may play a role in circadian rhythm behavior in some species. In any case, the identification of MagR finally explains how Cry modulates biological responses to magnetic fields.